Production of Alkali Bicarbonate and Alkali Hydroxide From Alkali Carbonate in an Electrolyte Cell.

ABSTRACT

Alkali bicarbonate is synthesized in an electrolytic cell from alkali carbonate. The electrolytic cell includes an alkali ion conductive membrane positioned between an anolyte compartment configured with an anode and a catholyte compartment configured with a cathode. The alkali conductive membrane selectively transports alkali ions and prevents the transport of anions produced in the catholyte compartment. An aqueous alkali carbonate solution is introduced into the anolyte compartment and electrolyzed at the anode to produce carbon dioxide and/or hydrogen ions which react with alkali carbonate to produce alkali bicarbonate. The alkali bicarbonate is recovered by filtration or other separation techniques. When the catholyte solution includes water, pure alkali hydroxide is produced. When the catholyte solution includes methanol, pure alkali methoxide is produced.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 61/320,204, filed Apr. 1, 2010, which application is incorporated by reference.

BACKGROUND

The invention relates to an electrolytic process to produce alkali bicarbonate, pure alkali hydroxide, and other useful products from alkali carbonate. Alkali carbonate is a readily available, low cost, alkali metal salt, most commonly in the form of sodium carbonate (Na₂CO₃) or soda ash. It would be advantageous to produce higher value products from alkali carbonate.

SUMMARY OF THE INVENTION

While the following disclosure relates to electrolysis products of a specific alkali carbonate, sodium carbonate (Na₂CO₃), it is understood that the disclosed invention relates to electrolysis of alkali carbonates in general, including but not limited to, sodium carbonate (Na₂CO₃), lithium carbonate (Li₂CO₃) and potassium carbonate (K₂CO₃). Where the following disclosure references sodium, it is understood that other alkali metals such as lithium and potassium may also be included.

Sodium carbonate (soda ash) can be used as a low cost starting material to make valuable products such as high purity sodium hydroxide (caustic soda) and baking soda (sodium bicarbonate). It is desirable that such a process uses very little energy input to render the conversion so that the process is economically viable. It is also desirable that pure and concentrated sodium hydroxide (up to 50 wt. %) and solid pure sodium bicarbonate can be recovered from impure sodium carbonate. The described invention accomplishes the stated objectives in an energy efficient manner such that the process is commercially attractive for preparation of valuable sodium compounds.

BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS

Embodiments of the present invention will be best understood by reference to the enclosed drawings. It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the methods and cells of the present invention, as represented in FIGS. 1 and 2, and is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention.

FIG. 1 discloses an electrolytic cell for conversion of alkali carbonate to alkali hydroxide and alkali bicarbonate.

FIG. 2 discloses a graph of flow rate and % CO₂ generated by electrolysis of sodium carbonate in a two compartment electrolytic cell as disclosed in Example 1.

FIG. 3 discloses a plot of pH verses time for the anolyte solution during electrolysis of sodium carbonate in the two compartment electrolytic cell operated in batch mode as disclosed in Example 1.

FIG. 4 discloses a ChemCAD predicted CO₂ solubility curve in sodium carbonate solutions.

FIG. 5 discloses a ChemCAD predicted CO₂ concentration in the anolyte during electrolysis.

FIG. 6 a discloses a ChemCAD predicted CO₂ amount in the anolyte vapor phase during electrolysis.

FIG. 6 b discloses a ChemCAD predicted rate of production of CO₂ based on the dated reported in FIG. 6 a.

FIG. 7 discloses a graph of voltage behavior of a two compartment electrolytic cell electrolyzing an aqueous sodium carbonate anolyte to produce sodium hydroxide and sodium bicarbonate as disclosed in Example 2.

FIG. 8 discloses a graph of current density and voltage for a NaSICON electrochemical cell with sodium carbonate (anolyte) and sodium hydroxide (catholyte) solutions as disclosed in Example 3.

FIG. 9 discloses a graph of sodium carbonate concentration and cell voltage over time in an electrolytic cell with 3.5 M Na₂CO₃ at 60° C. and operated at a constant current of 50 mA/cm², as disclosed in Example 4.

FIG. 10 discloses a graph of current efficiency generated from the FIG. 9 test data.

FIG. 11 discloses graphs of voltage and current density over time at 60° C. solution temperatures, as disclosed in Example 5.

FIG. 12 discloses graphs of the sodium carbonate anolyte solution and the sodium hydroxide catholyte solution over time, as disclosed in Example 5.

FIG. 13 discloses a process flow diagram for alkali ion removal from alkali carbonate solution and formation of concentrated caustic in the catholyte and alkali bicarbonate solid in the anolyte.

DETAILED DESCRIPTION OF THE INVENTION

Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. Additionally, while the following description refers to several embodiments and examples of the various components and aspects of the described invention, all of the described embodiments and examples are to be considered, in all respects, as illustrative only and not as being limiting in any manner.

Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are disclosed to provide a thorough understanding of embodiments of the invention. One having ordinary skill in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

The present invention provides an electrolytic process to produce alkali bicarbonate, alkali hydroxide, and other useful products from alkali carbonate. Such other products may include high purity alkali methoxide, oxygen, and hydrogen.

Referring now to FIG. 1, an electrolytic cell 100 according to one embodiment of the present invention is shown. The electrolytic cell 100 for conversion of alkali carbonate to alkali bicarbonate and alkali hydroxide uses an alkali ion conductive membrane 110 that divides the electrochemical cell 100 into two compartments: a first anolyte compartment 112 and a first catholyte compartment 114. An electrochemically active first anode 116 is housed in the first anolyte compartment 112 where oxidation reactions take place. An electrochemically active first cathode 118 is housed in the first catholyte compartment 114 where reduction reactions take place. The alkali ion conductive membrane 110 selectively transfers alkali ions (M⁺) 120, including but not limited to, sodium ions, lithium ions, and potassium ions, from the first anolyte compartment 112 to the first catholyte compartment 114 under the influence of an electrical potential 122 while preventing water or anion transportation from either compartment to the other side.

The alkali ion conductive membrane 110 can comprise virtually any suitable alkali ion conductive membrane that selectively conducts alkali ions and prevents the passage of water, hydroxide ions, or other reaction products. The alkali ion conducting membrane may comprise a ceramic, a polymer, or combinations thereof. In non-limiting embodiment, the alkali ion conducting membrane comprises an alkali ion super ion conducting (MSICON) membrane. Some non-limiting examples of such membranes include, but are not limited to, a NaSICON (sodium super ionic conductor membrane) and a NaSICON-type membrane. The alkali ion conductive membrane may be any of a number of sodium super ion conducting materials, including, without limitation, those disclosed in United States Patent Application Publications Nos. 2010/0331170 and 2008/0245671 and in U.S. Pat. No. 5,580,430. The foregoing applications and patent are hereby incorporated by reference. Where other non-sodium alkali carbonates are used within the scope of the present invention, it is to be understood that similar alkali ion conductive membranes such as a LiSICON membrane, a LiSICON-type membrane, a KSICON membrane, a KSICON-type membrane may be used. In some embodiments, an alkali ion conducting ion-exchange membrane may be used.

In one embodiment, the alkali ion conductive membrane is between about 200 microns and about 2000 microns thick. In other embodiment, the membrane is between about 400 and 1000 microns thick. In one embodiment 3 inch diameter MSICON wafers are assembled in a scaffold.

The anode 116 can comprise any suitable anode material that allows the cell to oxidize water or carbonate ions in the anolyte compartment when electrical potential passes between the anode and the cathode. Some non-limiting examples of suitable anode materials include, but are not limited to, platinum, titanium, nickel, cobalt, iron, stainless steel, metal alloys, mixed metal oxides (e.g. LaNiO₃), combinations thereof, and other known or novel anode materials. In one embodiment, the anode 116 may iron-nickel alloys such as KOVAR® or INVAR®. Additionally, in some embodiments the anode 116 comprises a dimensionally stable anode, which may include, but is not limited to, ruthenium dioxide and titanium dioxide on a titanium substrate, and ruthenium dioxide and tantalum pentoxide on a titanium substrate.

The cathode 118 may also be fabricated of any suitable cathode that allows the cell to reduce water or methanol in the catholyte compartment to produce hydroxide ions or methoxide ions and hydrogen gas. The cathode 118 may comprise the materials used in the anode 116. Some non-limiting examples of suitable cathode materials include, without limitation, nickel, stainless steel, graphite, KOVAR, and any other suitable cathode material that is known or novel.

The electrolytic cell 100 is operated by feeding an anolyte solution 124 into the first anolyte compartment 112. The anolyte solution is preferable an aqueous sodium carbonate solution. The anolyte solution 124 may be formed by taking solid sodium carbonate (which may or may not be impure) and preparing a concentrated sodium carbonate solution in water. The anolyte solution 124 may be prepared at a temperature between about 20° C. and about 70° C.

Under the influence of electric potential 122, electrochemical reactions take place at the anode 116 and cathode 118. Oxidation of water and/or sodium carbonate happens at the anode 116 resulting in the formation of oxygen gas and carbon dioxide as shown below:

H₂O→2H⁺+2e ⁻+½O₂  (1)

M₂CO₃→2M⁺+2e ⁻+CO₂+½O₂  (2)

In some disclosed embodiments, the species present in the anolyte 116 will then participate in chemical reactions in the electrolytic cell 100 as summarized below:

2H⁺+CO₃ ²⁻→H₂CO₃

H₂O+CO₂  (3)

CO₂+M₂CO₃+H₂O→2MHCO₃  (4)

H⁺+M₂CO₃→M⁺+MHCO₃  (5)

Reactions (4) and/or (5) happen when the pH of the anolyte reaches a value in the range of about 7 to about 11. In one embodiment, reactions (4) and/or (5) happen when the pH of the anolyte reaches a value in the range of about 7.5 to about 9.5. Reactions (4) and (5) also indicate that the extent of sodium bicarbonate formation will be dependent on the amount of dissolved carbon dioxide which in turn is a function of temperature and partial pressure of CO₂ in the gas phase. The concentration of sodium carbonate is also a contributing factor as the solution pH is a function of sodium carbonate concentration. During operation, the alkali ions (M⁺) 120 from the anolyte compartment 112 are transported across the alkali ion conductive membrane 110 to the catholyte compartment 114.

In some disclosed embodiments, the alkali bicarbonate formed in reactions (4) and (5), together with unreacted alkali carbonate solution, is withdrawn from the anolyte compartment 112 in stream 126. The alkali bicarbonate is recovered. One disclosed method of recovering the alkali bicarbonate is by precipitation and liquid/solid separation, such as filtration, centrifugation, and other similar processes. Alkali bicarbonate has a lower solubility in water compared to alkali carbonate making precipitation and liquid/solid separation feasible. Precipitation may be facilitated by cooling. The solubility of sodium carbonate is 28.7 wt. % at 30° C. compared to 9.9 wt. % for sodium bicarbonate at the same temperature.

In some disclosed embodiments, carbon dioxide 128 produced in the anolyte compartment 112 is withdrawn from the anolyte compartment and reacted directly with an alkali carbonate solution according to Reaction (4) in a separate reaction vessel. The alkali bicarbonate is recovered as described above.

In some disclosed embodiments, a catholyte solution feed stream 130 is fed into the catholyte compartment 114. In one disclosed embodiment, the catholyte solution comprises water. At least initially, the catholyte solution feed stream 130 preferably includes alkali ions, which may be in the form of an unsaturated alkali hydroxide solution. The concentration of alkali hydroxide is between about 0.1% by weight and about 50% by weight of the solution. In one embodiment, the catholyte solution feed stream 130 includes a dilute solution of alkali hydroxide. During operation, the source of alkali ions may be provided by alkali ions 120 transporting across the alkali ion conductive membrane 110 from the anolyte compartment 112 to the catholyte compartment 114. While alkali hydroxide is used in the following discussion, persons skilled in the art will appreciate that methanol may substitute alkali hydroxide in the apparatus for preparing alkali methylate instead. Thus, feed stream 130 may comprise methanol.

At the cathode 118, reduction of water to form hydrogen gas 132 and hydroxide ions takes place (Reaction 6). The hydroxide ions react with available alkali ions (M⁺) 120 (transported from anode compartment 112 via the alkali conductive membrane 110) to form alkali hydroxide as shown in Reaction (7). The alkali hydroxide may be recovered in catholyte product stream 134.

2H₂O+2e ⁻→H₂+2OH⁻  (6)

M⁺+2H₂O+2e ⁻→2NaOH+H₂  (7)

In the case of feed stream 130 comprising methanol, methoxide ions will react with available alkali ions to form alkali methoxide as shown in Reaction (8).

2M⁺+2CH₃OH+2e ⁻→2MOCH₃+H₂  (8)

The alkali methoxide may be recovered in catholyte product stream 134.

In one embodiment of the processes and apparatus of the present invention, the electrolytic cell 100 may be operated in a continuous mode. In a continuous mode, the cell is initially filled with anolyte solution and catholyte solution and then, during operation, additional solutions are fed into the cell and products, by-products, and/or diluted solutions are removed from the cell without ceasing operation of the cell. The feeding of the anolyte solution and catholyte solution may be done continuously or it may be done intermittently, meaning that the flow of a given solution is initiated or stopped according to the need for the solution and/or to maintain desired concentrations of solutions in the cell compartments, without emptying any one individual compartment or any combination of the two compartments. Similarly, the removal of solutions from the anolyte compartment and the catholyte compartment may also be continuous or intermittent. Control of the addition and/or removal of solutions from the cell may be done by any suitable means. Such means include manual operation, such as by one or more human operators, and automated operation, such as by using sensors, electronic valves, laboratory robots, etc. operating under computer or analog control. In automated operation, a valve or stopcock may be opened or closed according to a signal received from a computer or electronic controller on the basis of a timer, the output of a sensor, or other means. Examples of automated systems are well known in the art. Some combination of manual and automated operation may also be used. Alternatively, the amount of each solution that is to be added or removed per unit time to maintain a steady state may be experimentally determined for a given cell, and the flow of solutions into and out of the system set accordingly to achieve the steady state flow conditions.

In another embodiment, the electrolytic cell 100 is operated in batch mode. In batch mode, the anolyte solution and catholyte solution are fed initially into the cell and then the cell is operated until the desired concentration of product is produced in the anolyte and catholyte. The cell is then emptied, the products collected, and the cell refilled to start the process again. Alternatively, combinations of continuous mode and batch mode production may be used. Also, in either mode, the feeding of solutions may be done using a pre-prepared solution or using components that form the solution in situ.

It should be noted that both continuous and batch mode have dynamic flow of solutions. In one embodiment of continuous mode operation, the anolyte solution is added to the anolyte compartment so that the sodium concentration is maintained at a certain concentration or concentration range during operation of the electrolytic cell 100. In one embodiment of batch mode operation, a certain quantity of alkali ions are transferred through the alkali ion conductive membrane to the catholyte compartment and are not replenished, with the cell operation is stopped when the alkali ion concentration in the anolyte compartment reduces to a certain amount or when the appropriate product concentration is reached in the catholyte compartment.

The following examples are given to illustrate various embodiments within the scope of the present invention. These are given by way of example only, and it is understood that the following examples are not comprehensive or exhaustive of the many types of embodiments of the present invention that can be prepared in accordance with the present invention.

EXAMPLES Example 1 Measurement and Theoretical Analysis of Carbon Dioxide Generation from Electrolysis of Sodium Carbonate

The amount of gaseous CO₂ generated from the electrolysis of sodium carbonate anolyte was evaluated. Reaction (2) defined above is expected to occur at the anode 116 during electrolysis of sodium carbonate in a two compartment electrolytic cell 100 as shown in FIG. 1, utilizing a NaSICON membrane:

Na₂CO₃→2Na⁺+2e ⁻+CO₂+½O₂  (2)

Reaction (2) shows that for decomposition of every mole of sodium carbonate, one mole of carbon dioxide and half a mole of oxygen is generated. From reaction (2), the theoretical volume of carbon dioxide generated per minute was calculated at 33.6 cc/min (taking into account operating temperature and elevation) and oxygen was calculated at 16.8 cc/min. The expected total gas generation rate is 50.4 cc/min. The theoretical value of volume % of CO₂ expected based on Reaction (2) is 66.6% at the ratio of CO₂ to O₂ is 2:1.

Experimental Results

The carbon dioxide generated in the anolyte compartment was measured during electrolysis in batch mode operation with 2.5M Na₂CO₃ solution reduced to 0.85 M by continuous removal of sodium from sodium carbonate. See FIG. 7.

An IR-542 model IR sensor used to measure CO₂ was purchased from Detcon corporation with a maximum detection range of 0 to 100% v/v CO₂. a 50%±2% v/v CO₂/air gas mixture cylinder was used to calibrate the CO₂ sensor. The sensor measurement was 56% for the calibration mixture. A high precision flow meter was used to measure the flow rate.

The measured and expected (theoretical) flow rate and % CO₂ are shown in FIG. 2. The data recorded during daytime operation shows that during the first 4 hours the gas generation rate is much lower than theoretical (50.4 cc/min) and the % of CO₂ is also low compared to theoretical (66.6%). It is possible that the lower flow rate may be caused due to dissolution of the gases in the liquid phase. However even after 20 hours, the flow rate and % CO₂ remained the same indicating that the low flow rate may not be due to gas dissolution. During the measurements taken between 19 to 26 hours of cell operation which coincided with the appearance of sodium bicarbonate peak, the wt. % CO₂ and the flow rate started increasing. During the last 7 hours of the test the flow rate reached a value of 75 cc/min (greater than theoretical) and the % CO₂ had recorded a stable value of ˜70%. FIG. 2 discloses a graph of flow rate and % CO₂ generated by electrolysis of sodium carbonate in a two compartment electrolytic cell operated in batch mode.

The pH data is shown in FIG. 3. The data shows a pH of ˜11.7 for 2.5M Na₂CO₃. The pH drops to 9.5 at the 20 hour period and a value of 8.6 at 40 hour period.

Theoretical Modeling & Data Interpretation

In order to explain the experimental data, a theoretical analysis of the reactions in the anolyte was conducted. A basic simulation in ChemCAD to determine the concentrations of CO₂ and O₂ in the liquid and vapor phases at specific concentrations of Na₂CO₃ with varying amounts of CO₂ and O₂ feed rates was performed to construct the solubility curve of CO₂ wt. % versus Na₂CO₃ wt. % in aqueous solution. At a given concentration of Na₂CO₃, the initial feed rates of CO₂ and O₂ were varied until a vapor phase containing CO₂ was predicted by ChemCAD model. The case study for concentrations between 2.5M and 0.25M Na₂CO₃ was used to develop a solubility curve of CO₂ in Na₂CO₃ throughout the range of molarities associated with the actual test. FIG. 4 shows the solubility curve of CO₂ in aqueous Na₂CO₃ developed by ChemCAD. The CO₂ feed rate at which the vapor phase CO₂ appears at a given wt. % Na₂CO₃ is also shown.

FIG. 4 shows that as the wt. % of Na₂CO₃ decreases there is an increase in the dissolved concentration of CO₂. This seems counterintuitive if one assumes Na₂CO₃ is generating CO₂ throughout the electrolysis process, but is consistent with the formation of NaHCO₃ which consumes CO₂ as per Reaction (4) resulting in low amount of CO₂ in the liquid phase.

CO₂+Na₂CO₃+H₂O→2NaHCO₃  (4)

FIG. 4 also shows that as the weight percent of Na₂CO₃ increases additional CO₂ can be absorbed into the liquid face before appearing in the vapor phase. This also is consistent with NaHCO₃ production.

In an alternative ChemCAD analysis, by assuming an initial volume of 2.5M Na₂CO₃ solution the mass balance data of the anolyte was used to determine how the concentration of Na₂CO₃, O₂ and CO₂ would theoretically change over time. Every 2 hours the associated masses of each substance was entered into ChemCAD to determine the concentrations of each substance present (Na₂CO₃, O₂, CO₂, and NaHCO₃) in the liquid and gas phase. This data was then used to create plots to map concentration of various chemicals vs. time as the reactions proceeded.

The concentration of CO₂ and O₂ vs. time in the liquid phase is shown in FIG. 5. The data shows that essentially no CO₂ is present in the liquid phase (consequently in vapor phase) for ˜1,100 minutes (˜18.33 hours).

FIG. 6 a shows that after about 1200 minutes CO₂ starts appearing in the vapor phase and the amount in vapor phase increases linearly. This observation matches very well with the experimental data shown in FIG. 2 where minimal % of CO₂ was observed during the start of the test up to 20 hrs. FIG. 6 b shows the trend line for the data points after 1200 minutes, which has a slope of 0.0902 g/min. Therefore, the rate that CO₂ is added to the vapor phase is 0.0902 g/min. This rate is exactly double the predicted rate of CO₂ formation from the mass balance for decomposition of sodium carbonate from Reaction (2). This increased generation rate can be explained by decomposition reaction of sodium bicarbonate which generates a mole of CO₂ per every mole of Na⁺ transferred unlike sodium carbonate where half a mole of CO₂ per every mole of Na⁺ transferred (Reaction (9)).

NaHCO₃ →e ⁻+Na⁺+CO₂+½H₂O+¼O₂  (9)

The higher than theoretically predicted flow rate (75 cc/min compared to 50.4 cc/min) seen after 40 hours in the experiment (FIG. 2) is therefore explained by the decomposition reaction of sodium bicarbonate and not sodium carbonate.

Table 1 compares the theoretical prediction of flow rate data based on Na₂CO₃ decomposition only (Reaction (2)) to the actual experimental data and the predictions of ChemCAD. The values in Table 1 show that ChemCAD predictions match the experimental results much more than theoretical prediction based on Na₂CO₃ decomposition.

TABLE 1 Theoretical prediction Experimental ChemCAD based upon Na₂CO₃ Observation Prediction Time Period (hr) decomposition (cc/min) (cc/min) (cc/min) 0-4 50.4 10-20 0 19-26 50.4 30-46 75.6 40-47 50.4 75 75.6

CONCLUSIONS

The experimental out-gas flow rate during batch-mode electrolysis of 2.5M Na₂CO₃ solution is 10-20 cc/min during first 4 hours, 30-46 cc/min between 19 and 26 hours and reached a maximum of 75 cc/min between 40 and 47 hours. The experimental volume % CO₂ recorded during batch-mode electrolysis of 2.5M Na₂CO₃ solution is 4% during first 4 hours, 6.3-16% between 19 and 26 hours and reached a maximum of 70% between 40 and 47 hours. These results differ from that predicted by simple Na₂CO₃ decomposition only.

ChemCAD analysis predicted that the CO₂ evolved during first 20 hours of the electrolysis causes the formation of NaHCO₃ (or bicarbonate ions due to lowering of pH during the process) and no CO₂ is present in the vapor phase. This prediction matches well with the experimental data. ChemCAD predicted that as the electrolysis continues, the NaHCO₃ (or HCO₃ ⁻ ions) are electrolyzed resulting in the formation of higher than expected amount of CO₂ in the vapor phase. This prediction matches well with the experimental data.

The experimental results for the amount of CO₂ evolved during 2.5 M sodium carbonate batch electrolysis matched well with the predictions from ChemCAD theoretical analysis.

Example 2 Sodium Bicarbonate and Sodium Hydroxide Generation from Electrolysis of Sodium Carbonate

The production of sodium bicarbonate and sodium hydroxide from sodium carbonate was accomplished in an electrolytic cell 100 as shown in FIG. 1. The electrolytic cell used a NaSICON sodium ion conducting ceramic membrane was assembled with a Pt/Ti anode and a Ni cathode. A 2.88 M sodium carbonate anolyte solution and a catholyte solution at 15 wt % sodium hydroxide were prepared. The solutions were heated to temperature of 65° C. and then circulated through respective compartments. The cell was operated at a current density of 50 mA per cm² of membrane. At the cell a voltage measurement was made. The corresponding data is represented in FIG. 7.

Referring now FIG. 7 voltage and current are shown as a function of time. FIG. 7 shows that the cell was operated in batch mode for a period of 47.5 hours during which the anolyte sodium carbonate concentration decreased to 0.85 M. The voltage increased between 20 to 33 hours. This behavior indicates that the resistance within the cell had increased. Examination of the cell interior showed white precipitate on the surface of the electrodes, membranes, and other cell components. Analysis of white precipitate showed that the material was sodium bicarbonate. This increase in voltage was due to precipitation of sodium bicarbonate on the anode or the membrane surface which was temporary and the cell recovered during further operation as indicated by lowering of voltage from 33 hours to 40 hours. The bicarbonate ions converted back into soluble carbonate during this time of voltage and pH decrease. The higher voltage above 40 hours was due to decrease in conductivity of the anolyte solution because of lower concentration of sodium carbonate. During the process, the catholyte sodium hydroxide concentration increased from 4.2M to 8.26M. This test showed a process efficiency of 100% indicating that nearly theoretical amount of sodium as predicted by Faraday's law was transferred from Na₂CO₃.

The experimental observations are supported by Table 2 shown below where the mole fraction of carbonic acid, bicarbonate, and carbonate species in an aqueous solution as a function of pH are given.

TABLE 2 MOLE FRACTION (25° C.) pH [H₂CO₃] [HCO₃ ⁻] [CO₃ ²⁻] 2.0 1.000 0 — — 2.5 0.999 9 0.000 1 — 3.0 0.999 6 0.000 4 — 3.5 0.998 6 0.001 4 — 4.0 0.995 7 0.004 3 — 4.5 0.986 6 0.013 4 — 5.0 0.958 7 0.041 3 — 5.5 0.880 0 0.120 0 — 6.0 0.698 8 0.301 2 0.000 0 6.5 0.423 2 0.576 7 0.000 1 7.0 0.188 3 0.811 3 0.000 4 7.5 0.068 3 0.930 3 0.001 4 8.0 0.022 6 0.972 8 0.004 6 8.5 0.007 2 0.978 3 0.014 5 9.0 0.002 2 0.953 0 0.044 8 9.5 0.000 6 0.870 1 0.129 3 10.0 0.000 2 0.680 1 0.319 7 10.5 0.000 0 0.402 2 0.507 8 11.0 — 0.175 4 0.824 6 11.5 — 0.063 0 0.937 0 12.0 — 0.020 8 0.979 2 12.5 — 0.006 7 0.993 3 13.0 — 0.002 1 0.997 9

The starting pH of the 2.88M sodium carbonate solution used in Example 2 was nearly 13 and so the solution contained predominantly carbonate ions. As sodium was removed and carbon dioxide released from the anolyte, the pH of the solution decreased and more sodium bicarbonate formed. At a pH of ˜8.5, the solution contained mainly bicarbonate ions. The sodium bicarbonate precipitated out as its solubility was lower than that of sodium carbonate. The precipitated sodium bicarbonate formed a resistive coating on the anode and alkali ion selective membrane causing the increase in cell voltage. Further sodium removal decreased the pH further converting the bicarbonate ions back to soluble carbonate ions. The resistive coating on the anode and membrane was removed and the cell voltage decreased.

The data presented above is supported by theory and conclusively shows that sodium hydroxide and sodium bicarbonate are produced by the electrolytic decomposition of sodium carbonate.

Example 3 Operation of Electrolytic Cell Containing Sodium Carbonate and Sodium Hydroxide

The operation of an electrolytic cell containing sodium carbonate in the anolyte compartment and sodium hydroxide in the catholyte compartment was tested. The electrolytic cell included three inch diameter NaSICON material discs that were face sealed to create an alkali ion conductive membrane. The anode comprised Pt/Ti and the cathode comprised Ni. The anolyte compartment was filled with a 28.5 wt % sodium carbonate solution. The catholyte compartment was filled with a 15 wt % sodium hydroxide solution. The solutions were heated to temperature and then a voltage was applied to the cell. The current density was measured at different cell voltages. The corresponding measured data is represented in FIG. 8.

The current density and voltage plot of FIG. 8 shows promise for successful transport of sodium and evolution of carbon dioxide. The cell voltage is reasonable for corresponding current densities compared to a caustic anolyte and a caustic catholyte cell.

Example 4 Operation Of Electrolytic Cell Containing Sodium Carbonate And Sodium Hydroxide at Higher Temperatures

The operation of an electrolytic cell containing sodium carbonate in the anolyte compartment and sodium hydroxide in the catholyte compartment was tested. The electrolytic cell included three inch diameter NaSICON material discs that were face sealed to create an alkali ion conductive membrane. The anode comprised Pt/Ti and the cathode comprised Ni. The anolyte compartment was filled with a 3.5M sodium carbonate solution. The catholyte compartment was filled with a 15 wt % sodium hydroxide solution. The solutions were heated to 60° C. temperature and then a constant current density of 50 mA/cm² was applied to the cell. The sodium carbonate concentration was measured over time to determine current efficiency of the NaSICON membrane. FIG. 9 shows a graph of the sodium carbonate concentration and cell voltage over time. FIG. 10 shows a graph of current efficiency generated from the FIG. 9 test data.

Precipitation of solids formed on cell components which resulted in an increase in cell voltage. Nevertheless, the test results show that the Na₂CO₃ could be dissolved and thus more controlled and easily removed by increasing the temperature to 70° C. and adding water. The NaSICON membrane showed no sign of degradation or failure. Current efficiency was also measured as shown in FIG. 10. The current efficiency was above 95% for most of the test. The decrease in current efficiency could be from un-dissolved precipitates that were not accounted for in the total sodium in the anolyte. The results show that Na₂CO₃ may be processed in an electrolytic cell with a NaSICON membrane at high current efficiencies with no degradation of the NaSICON membrane.

Example 5 Sodium Bicarbonate and Sodium Hydroxide Generation from Electrolysis of Sodium Carbonate at Higher Temperature and Lower Starting Na₂CO₃ Concentration (2.4 M)

The operation of an electrolytic cell containing sodium carbonate in the anolyte compartment and sodium hydroxide in the catholyte compartment was tested. The electrolytic cell included three inch diameter NaSICON material discs that were face sealed to create an alkali ion conductive membrane. The anode comprised Pt/Ti and the cathode comprised Ni. The anolyte compartment was filled with a 2.4M sodium carbonate solution. The catholyte compartment was filled with a 15 wt % sodium hydroxide solution. The solutions were heated to 60° C. temperature and then a constant current density of 50 mA/cm² was applied to the cell. The sodium carbonate concentration was measured over time to determine current efficiency of the membrane. FIG. 11 discloses graphs of voltage and current density over time. FIG. 12 discloses graphs of the sodium carbonate anolyte solution and the sodium hydroxide catholyte solution over time.

This example shows control of the precipitation of sodium bicarbonate that occurred during the test. At the point that precipitation occurred, the solution was heated to 64° C. Upon heating the solution, the precipitate dissolved without any issues except for a small increase in cell voltage which did return to normal. At about 120 hours, the concentration of the anolyte solution was such that the resistance increased resulting in an increase in cell voltage. Since the power supply being used was limited to about 25 volts, the limit was reached which automatically decreased the current density to the cell as seen in FIG. 11. Since the current was not held constant between about 120 to 160 hours, current efficiency could not be accurately determined. The current efficiency for most of the test showed excellent performance by maintaining the efficiency above 95%.

In this example, sodium bicarbonate was consistently created at temperatures of about 60° C. and lower than 64° C. It was observed that operating the cell at a higher starting Na₂CO₃ concentration and operating at 60° C. or lower appeared to increase the rate at which sodium bicarbonate forms.

FIG. 13 shows a process flow diagram 200 for a process to electrolyze sodium carbonate solution and form concentrated caustic (NaOH) in the catholyte compartment and sodium bicarbonate in the anolyte compartment. The process of making alkali bicarbonate and alkali hydroxide from alkali carbonate solution in an electrolytic cell includes preparing anolyte and catholyte feed solutions. The anolyte feed 210 may include a concentrated aqueous alkali carbonate or in the disclosed example, sodium carbonate. The alkali carbonate may be an impure source, that is, containing other cations such as magnesium and calcium. The catholyte feed 212 may include water and optionally dilute alkali hydroxide, such as sodium hydroxide. A two compartment electrolytic cell 214 includes an alkali ion conducting membrane, such as NaSICON, separating the anode and cathode compartments. The electrolytic cell 214 may be of the type described in FIG. 1.

The anolyte and catholyte feed solutions are fed into anode and cathode compartments respectively. The process continues by electrolyzing the solutions. The process may be operated continuously or in batch mode. During electrolysis, alkali ions are transferred from the anolyte compartment to the catholyte compartment. Alkali bicarbonate forms in the anolyte compartment and concentrated alkali hydroxide is formed in the catholyte compartment. The anolyte solution 216, comprising alkali bicarbonate and unreacted alkali carbonate 216, is removed from the anolyte compartment. The catholyte solution 218, comprising concentrated alkali hydroxide 218, is recovered from the catholyte compartment. On significant benefit resulting from the use of an alkali ion selective membrane, such as a NaSICON membrane, in the electrolytic cell 214 is the ability to produce pure sodium hydroxide in the catholyte compartment. Impurities, such as magnesium or calcium, cannot pass through the NaSICON membrane, thereby permitting the production of pure sodium hydroxide.

The electrolytic cell 214 may be operated at various temperature values to vary the alkali bicarbonate to alkali hydroxide product ratio. In one embodiment, the temperature range may be from about 5° C. to about 75° C. In some embodiments the operating temperature is less than 75° C. In other embodiments, the operating temperature is less than 70° C. In some embodiments, the operating temperature is less than 65° C. In yet other embodiments, the operating temperature is less than 60° C. In another embodiment, the operating temperature may be greater than about 20° C. In other embodiments, the operating temperature may be greater than about 30° C. In still other embodiments, the operating temperature may be greater than about 35° C. In other embodiments, the temperature is selected so as to maximize the production of the alkali bicarbonate and still maintain economical voltages. In another embodiment, the temperature is chosen depending upon whether more alkali bicarbonate or more alkali hydroxide is desired. The cell operation temperature may be such that the ratio of solubility of sodium carbonate compared to that of sodium bicarbonate is high (˜40° C.).

The electrolytic cell 214 may be operated at various pressure values in the anolyte compartment to vary the alkali bicarbonate to alkali hydroxide product ratio. In one embodiment, the pressure in the anode compartment may be from about 0.1 atmospheres to about 5 atmospheres. In other embodiments, the pressure in the anode compartment is about 3 atmospheres. In one embodiment the pressure is chosen to facilitate efficient operation of the electrolytic cell. In another embodiment, the pressure is chosen depending upon whether more alkali bicarbonate or more alkali hydroxide is desired. A pressurized anolyte compartment promotes carbon dioxide dissolution and reaction to form bicarbonate.

In one embodiment, solid alkali bicarbonate 220 may be recovered from the anolyte solution 216 by filtration or separation 222. Such filtration or separation techniques may include filtering or settling or applying other solid/liquid separation techniques on the anolyte solution. Cooling the solution may facilitate removal of the alkali bicarbonate due to its lower water solubility at low temperature compared alkali carbonate. As shown in FIG. 13, the alkali bicarbonate may be sodium bicarbonate and the alkali hydroxide may be sodium hydroxide.

Following removal of the alkali bicarbonate 220, the supernatant may comprise a dilute alkali carbonate solution 224. Solid alkali carbonate 226 may be added to the dilute alkali carbonate to form the concentrated alkali carbonate solution used as the anolyte feed solution 210.

During operation of the electrolytic cell 214, certain gaseous products may be produced. For example, oxygen and optionally carbon dioxide 228 may be formed in the anolyte compartment and recovered. Hydrogen 230 may be formed in the catholyte compartment and recovered.

While specific embodiments and examples of the present invention have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying claims. 

1. A process for synthesizing alkali bicarbonate comprising: providing an electrolytic cell comprising an alkali ion conductive membrane positioned between an anolyte compartment configured with an anode and a catholyte compartment configured with a cathode, said alkali conductive membrane being configured to selectively transport alkali ions and prevent the transport of anions produced in the catholyte compartment; introducing aqueous alkali carbonate into the anolyte compartment; electrolyzing aqueous alkali carbonate at the anode according to one or both of the following reactions: H₂O→2H⁺+2e ⁻+½O₂ M₂CO₃→2M⁺+2e ⁻+CO₂+½O₂ wherein M is an alkali metal selected from Li, Na, and K; reacting either or both of the H⁺ or CO₂ produced above with aqueous alkali carbonate according to one or both of the following reactions: CO₂+M₂CO₃+H₂O→2MHCO₃(precipitate) H⁺+M₂CO₃→M⁺+MHCO₃(precipitate) and recovering the alkali bicarbonate produced.
 2. The process for synthesizing alkali bicarbonate according to claim 1, further comprising the step of maintaining the pH in the anolyte compartment at a pH in the range of about 7 to about
 11. 3. The process of claim 2, further comprising the step of maintaining the pH in the anolyte compartment at a pH in the range of about 7.5 to about 9.5.
 4. The process for synthesizing alkali bicarbonate according to claim 2, wherein the pH in the anolyte compartment is maintained by controlling the concentration of alkali carbonate introduced into the anolyte compartment and the cell operation temperature.
 5. The process for synthesizing alkali bicarbonate according to claim 1, further comprising the step of maintaining the temperature in the anolyte compartment at a temperature in the range of 20° C. to 70° C.
 6. The process for synthesizing alkali bicarbonate according to claim 1, further comprising: reacting CO₂ with aqueous alkali carbonate outside the anolyte compartment.
 7. The process for synthesizing alkali bicarbonate according to claim 5, further comprising: using the CO₂ recovered from the anolyte compartment; and reacting that CO₂ with aqueous alkali carbonate outside the anolyte compartment.
 8. The process for synthesizing alkali bicarbonate according to claim 1, further comprising: introducing water and optionally a dilute alkali hydroxide solution into the catholyte compartment; electrolyzing the water at the cathode according to the following reaction: 2M⁺+2H₂O+2e ⁻→2MOH+H₂ and recovering the alkali hydroxide and hydrogen produced.
 9. The process for synthesizing alkali bicarbonate according to claim 1, further comprising: introducing methanol into the catholyte compartment; electrolyzing the methanol solution at the cathode in the presence of alkali ions transported from the anolyte compartment across the alkali ion conductive membrane according to the following reaction: 2M⁺+2CH₃OH+2e ⁻→2MOCH₃+H₂ and recovering the alkali methoxide and hydrogen produced.
 10. The process for synthesizing alkali bicarbonate according to claim 1, wherein the alkali ion conductive membrane is an alkali ion super ion conductive membrane selected from NaSICON or NaSICON-type membranes, LiSICON or a LiSICON-type membranes, and KSICON or KSICON-type membranes.
 11. The process for synthesizing alkali bicarbonate according to claim 1, further comprising pressurizing the anolyte compartment to promote carbon dioxide dissolution and reaction to form bicarbonate.
 12. The process for synthesizing alkali bicarbonate according to claim 1, wherein the alkali metal is sodium.
 13. A process for synthesizing pure alkali hydroxide from alkali carbonate comprising: providing an electrolytic cell comprising an alkali ion conductive membrane positioned between an anolyte compartment configured with an anode and a catholyte compartment configured with a cathode, said alkali conductive membrane being configured to selectively transport alkali ions and prevent the transport of anions produced in the catholyte compartment; introducing aqueous alkali carbonate into the anolyte compartment; electrolyzing aqueous alkali carbonate at the anode according to one or both of the following reactions: H₂O→2H⁺+2e ⁻+½O₂ M₂CO₃→2M⁺+2e ⁻+CO₂ +½O ₂ wherein M is an alkali metal selected from Li, Na, and K; introducing water and optionally a dilute alkali hydroxide solution into the catholyte compartment; electrolyzing the water at the cathode according to the following reaction: 2M⁺+2H₂O+2e ⁻→2MOH+H₂ and recovering the pure alkali hydroxide produced.
 14. The process for synthesizing pure alkali hydroxide from alkali carbonate according to claim 13, further comprising recovering the hydrogen produced.
 15. The process for synthesizing pure alkali hydroxide from alkali carbonate according to claim 13, further comprising: reacting either or both of the H⁺ or CO₂ produced above with aqueous alkali carbonate according to one or both of the following reactions: CO₂+M₂CO₃+H₂O→2 MHCO₃(precipitate) H⁺+M₂CO₃→M⁺+MHCO₃(precipitate) and recovering the alkali bicarbonate produced.
 16. The process for synthesizing pure alkali hydroxide from alkali carbonate according to claim 13, wherein the alkali ion conductive membrane is an alkali ion super ion conductive membrane selected from NaSICON or NaSICON-type membranes, LiSICON or a LiSICON-type membranes, and KSICON or KSICON-type membranes.
 17. The process for synthesizing alkali bicarbonate according to claim 13, wherein the alkali metal is sodium.
 18. A process for synthesizing pure alkali methoxide from alkali carbonate and methanol comprising: providing an electrolytic cell comprising an alkali ion conductive membrane positioned between an anolyte compartment configured with an anode and a catholyte compartment configured with a cathode, said alkali conductive membrane being configured to selectively transport alkali ions and prevent the transport of anions produced in the catholyte compartment; introducing aqueous alkali carbonate into the anolyte compartment; electrolyzing aqueous alkali carbonate at the anode according to one or both of the following reactions: H₂O→2H⁺+2e ⁻+½O₂ M₂CO₃→2M⁺+2e ⁻+CO₂+½O₂ wherein M is an alkali metal selected from Li, Na, and K; introducing methanol into the catholyte compartment; electrolyzing the methanol solution at the cathode in the presence of alkali ions transported from the anolyte compartment across the alkali ion conductive membrane according to the following reaction: 2M⁺+2CH₃OH+2e ⁻→2MOCH₃+H₂ and recovering the pure alkali methoxide produced.
 19. The process for synthesizing pure alkali methoxide from alkali carbonate and methanol according to claim 18, further comprising recovering the hydrogen produced.
 20. The process for synthesizing pure alkali methoxide from alkali carbonate and methanol according to claim 18, further comprising: reacting either or both of the H⁺ or CO₂ produced above with aqueous alkali carbonate according to one or both of the following reactions: CO₂+M₂CO₃+H₂O→2MHCO₃(precipitate) H⁺+M₂CO₃→M⁺+MHCO₃(precipitate) and recovering the alkali bicarbonate produced.
 21. The process for synthesizing pure alkali methoxide from alkali carbonate and methanol according to claim 18, wherein the alkali ion conductive membrane is an alkali ion super ion conductive membrane selected from NaSICON or NaSICON-type membranes, LiSICON or a LiSICON-type membranes, and KSICON or KSICON-type membranes.
 22. The process for synthesizing pure alkali methoxide from alkali carbonate and methanol according to claim 18, wherein the alkali metal is sodium. 